So-called plunger type locks are commonly used in the utility industry to lock gas valves, meter housing and the like. Such locks have an unlocking plunger recessed within a housing, said plunger being normally biased to the locking position, and may be pulled by a suitable tool or "key" to the unlocking position.
One type of plunger lock that has been widely used is illustrated in U.S. Pat. No. 3,186,196, issued June 1, 1965. In the lock disclosed therein, the end of the plunger is formed to provide a hollow pin, recessed in a housing opening, as the unlocking member. The pin is biased away from the housing opening to the unlocking position, and may be pulled toward the opening to an unlocking position. The pin is pulled to the unlocking position by a suitable tool, commonly called a "key" which has an elongated mechanism for inserting through the opening into the hollow pin. Movement of an operating handle on the tool causes longitudinal movement of one portion of the elongated mechanism in relation to the other portion, causing a lateral spreading of the end of the mechanism, which thereby frictionally engages the inside of the hollow pin. Thus rearward movement of the mechanism causes the pin to move rearwardly to the unlocking position. One form of such expandable mechanism is shown in the above-mentioned U.S. Pat. No. 3,186,196, and another form is shown in U.S. Pat. No. 3,446,045, issued May 27, 1969.
However, a plunger lock with a new form of pin has begun to achieve commercial success. This new type of lock utilizes a solid pin on the end of the plunger as the unlocking member, and a tool is provided that grasps the external surface of the pin for pulling the pin to the unlocking position.
A lock having this structure is shown in U.S. Pat. No. 4,015,456, issued Apr. 5, 1977, and one form of tool that may be used with this type of pin is shown in U.S. Pat. No. 4,058,992 issued Nov. 22, 1977.
It has been found that utilities that already use substantial quantities of the plunger lock with the hollow pin are reluctant to purchase the new type of lock with the solid pin, since they would then have to equip their employees with a new type of key to operate the new type of lock, and the employees would have to carry both types of keys in servicing meters or in locking and unlocking valves.